From the early days of radio, the non-ideality of the Earth's atmosphere as a transmission medium has been apparent. Radiated energy does not travel uniformly from the transmitter through the atmosphere to present a unified wave front to the receiver. On the contrary, portions of the radiated energy are reflected in many different directions by features of the terrain, non-uniformities in the atmosphere itself, and by man-made structures. Different portions of the radiated energy reach the receiver at different times along different paths resulting in a time-varying multi-path characteristic that produces, alternatively, destructive and constructive interference.
This multi-path characteristic causes a transmitted radio pulse to undergo different gain and phase variations resulting in reception of a distorted pulse. Pulse distortion from the channel medium causes interference between adjacent samples of the received signal resulting in a phenomenon known as inter-symbol interference. Inter-symbol interference can be viewed as a smearing of the transmitted pulse by the multi-path, causing overlap between successive pulses. Interference with a particular pulse can occur as a result of both past and future pulses, since the pulse is detected at the receiver after a mean path delay when the pulse is received with greatest strength. Portions of pulse energy from a past pulse that have experienced a greater than average delay may therefore interfere with a subsequent pulse, and portions of energy from a "future" pulse, i.e., a pulse that the radio receiver is not ready to detect, experiencing a less than average path delay may interfere with the present pulse being detected by the radio receive.
As is known in the art, inter-symbol interference may be greatly reduced using an adaptive equalizer. In one type of equalizer, for example, time-shifted versions of the received signal are combined with the received signal according to appropriate weights to cancel future and past interference. Such an equalizer may be constructed using a transverse filter including a tapped delay line. The appropriate weights according to which delayed signal replicas are combined with the presently received signal are arrived at during what is known as equalizer training. During equalizer training, a known sequence of symbols is transmitted and the output of the equalizer in response to the known sequence of symbols is compared to the known sequence itself. In an iterative process, an adaption algorithm optimized to minimize the difference between the detected symbol sequence and the actual symbol sequence sets the weights of the equalizer. If the weights of the equalizer are properly set, the error will be reduced to a minimum level and the equalizer will be said to have converged. If for some reasons the weights are not properly set, the error may actually increase and the equalizer will be said to have diverged.
In another well-known type of equalizer, the Viterbi equalizer, a set of complex-valued weights representing the impulse response of the communications channel is derived using a transverse filter in substantially the same manner as described above. Using the channel estimate, received pulses are detected according to the Viterbi algorithm to yield symbols of maximum likelihood given the channel estimate.
Radio channel conditions do not remain constant from one time to the next but are always changing according to atmospheric conditions as affected by the weather, etc. Accordingly, the equalizer weights have to be continually updated. Radio channel conditions change especially rapidly in a mobile radiotelephone system wherein the location of the receiver relative to the transmitter is constantly changing, perhaps at a high rate. Frequent retraining of the equalizer therefore becomes important in order to maintain sufficient reception quality.
At the same time, the transmission of known training sequences in the communications stream displaces the desired communications between the transmitter and receiver so as to increase transmission overhead. Transmission overhead reduces the effective available communications bandwidth. Bandwidth is of course a precious resource in a radiotelephone system.
Accordingly, what is needed is a way of frequently retraining an equalizer in a radio telecommunications system while minimizing transmission overhead.